The present invention relates to determining the characteristics of a metallurgical product surface coating, in particular in-line, during the feed of the said product during manufacture. It more especially concerns the determining of galvanisation coating characteristics on steel strips in particular of type known under the trade name of GALVALLIA® sheets.
It is recalled here that these sheets are galvanised strips which are submitted to a heat treatment called alliation the aim of which is to provide these strips with improved surface characteristics for example allowing better paint adhesion or offering better stampability.
This alliation treatment is a heat treatment performed on the galvanised strip in an alliation tower with continuous galvanised strip feed. The object of the heat treatment is to ensure diffusion of the iron in the underlayer towards the surface to attain the objectives mentioned above. Typically, the alliation percentage is around 10% in iron concentration in relation to the zinc.
However, at present, the required alliation level, that is the relative extent of this diffusion, varies considerably from one user of the said strips to another and it is therefore important to be able to ensure during manufacture a continuous check of the alliation level. To meet the requests of the customers using the said coated strips, it is therefore necessary to modify during manufacture the adjustment parameters of the installation which performs the alliation heat treatment, that is, mainly the treatment temperature and time.
At present, no system can check in real time and in-line that the GALVALLIA® surface characteristics are those required. Indeed, it is known that alliation causes significant modifications to the surface properties in physicochemical and microgeometrical terms. At present, the alliation level is determined only in laboratory by measuring powdering, iron content or again by metallographical analysis enabling the coating surface crystals to be characterised.
Moreover, to obtain an evaluation as accurate as possible of the surface alliation, it is also necessary to overcome, during the measurement, morphological variations, such as micro-roughness or equivalent geometrical characteristics to ensure sensitivity only to the physicochemical variations of the surface. These physicochemical variations of the extreme surface in fact reflect the migration of the iron atoms of the coating to the extreme surface, which migration corresponds to the alliation level of the product.
As these checks are deferred in relation to production, defects can appear on the strips, such as, for example, powdering or separation of the coating due to an excessive alliation level.